There are many known communications systems which use encoded data from which timing and data signals may be derived. In one form, a data communication network may consist of a ring-like signal path including one or more active repeaters and use a continuously circulating token to mediate access. For example, see the network described in Clark, Pogran and Reed, Proc. IEEE 66, pp. 1497-1516. In such networks, a plurality of user terminals are coupled to a ring. The ring is normally quiet (in the absence of use) while a digital "token" circulates around the ring. Upon detection of the token at his terminal, a user may "grab" the token and thereby gain access to the entire bandwidth of the ring. The user then transmits a message followed by re-insertion of a token onto the ring. Any other user may then gain access to the ring when he detects the token at his terminal.
In practice, the token may recirculate many times before a user may desire access to the ring. However, as the token recirculates, low level noise adds to that signal. The resultant token-plus-noise appears to a detector (for example, at a user terminal) as a digital signal with phase jitter, i.e. the transitions between digital states appear delayed or advanced in time from their nominal positions. Particularly for relatively short tokens it is necessary to correct for the corrupting influence of this random noise on the timing of the pules that constitute the digital signals. To this end, rings in such a digital communication networks include active repeaters for restoring the digital signal on the ring to nominally correct timing.
For a token-mediated ring, there are two modes of operation, originating a message and circulating a token. When originating a message, one user terminal breaks the ring (after capturing the token), transmits its message into the ring followed by the token, and awaits return of the message on the receive side, and then drains the message from the ring. In this mode, each transmitted bit is repeated once by each station of the ring, and one can calculate the expected accumulation of timing noise as the bits progress around the ring. The designer can then choose parameters of signal levels so as to assure that every station will be able to receive the message with high probability. Thus, in this mode, since the message only goes around the ring once and the phase jitter may be accommodated, no timing restoration need be done by the repeaters.
When circulating a token, however, the token bit pattern, once introduced to the ring, circulates around and around the ring until such time as some user terminal decides to originate a message; the token bit pattern may go through many millions of cycles of detection and retransmission. In this case, no choice of signal-to-noise ratio can prevent the token timing from eventually being degraded to unrecognizability, and some timing restoration measure is necessary.
In the prior art, timing regeneration may be accomplished by introducing new, corrected timing on every repeated bit. In that scheme, every repeater has its own independent clock that is used for transmission of the repeated signal. As successive repeaters will have clocks that operate at slightly different frequencies, it is necessary to introduce occasional time wedges in some bits to maintain approximate phase match between the received and the regenerated signal. That system uses a timing clock that is some multiple of the data transmission rate (e.g. six times) and the speed of the circuitry using that clock limits the maximum frequency of transmission. In addition, the cumulative effect of timing wedges applied to a continuously circulating token must be somehow controlled.
A second alternative approach requires the synchronization of the clocks of the ring of repeaters, using a phase-locked-loop and voltage-controlled oscillator for the timing clock at each repeater. This approach requires continuous transmission of timing bits to maintain loop lock which in turn requires a closed loop circulation delay of an integral number of bit times. It also requires careful analysis and design to assure stability of the ring of phase-locked-loops and phase-delay compensators. The analog circuitry required to obtain frequency lock and correct phase delay is relatively complex, and is generally concentrated in a special station, as is done for example in The Cambridge University ring (Wilkes and Wheeler, Proc. Local Area Comm. Network Symp., pp. 47-60.) and the Century Data Bus (Okuda, Kunikyo, and Kaji, Proc. Fourth Int. Conf. on Computer Comm., pp. 161-166).
It is an object of the present invention to provide a ring communications network having improved data regeneration.